The present invention relates generally to the field of concrete manufacture and more particularly to the field of recovery methods for recycling hydrated cement from returned concrete.
Over the past decade, governments and environmental groups have increased pressure on the ready-mix industry to reduce waste discharge. The high-pH, toxic, alkaline run-off caused by waste cement is now classified as a hazardous waste in some parts of the world and it is expected that the U.S. and Canada will soon mandate zero-discharge regulations for its ready-mix industry.
The problem is caused by hydrated cement, which contains Calcium Hydroxide-(Ca(OH)2), the highly alkaline substance that acts as the binary agent in concrete. When Ca(OH)2 is released into the environment, it can be deadly to fish and wildlife and can potentially poison other public waterways. Ca(OH)2 created by concrete production routinely exceeds the maximum allowed discharge pH levels for most civic process discharge permits.
Conversely, if recycled into fresh concrete without treatment, Ca(OH)2 can cause poor slump control, reduced strength and unpredictable finishing characteristics. Once cement is exposed to water and hydration begins, it must continue until the process is exhausted or suspended. The longer the hydration period, the greater amount of Ca(OH)2 produced.
In recent years, chemical admixture producers have developed hydration stabilization admixtures (HSA), which have provided the solution to the hydration problem of the cement. Since it is now possible to suspend hydration for a controlled period of time (stabilize), partially hydrated cement can be recycled before it entirely converts to calcium hydroxide through hydration. This also means that a portion of the cementitious value can be saved for later use.
Conventionally when a concrete mixer truck returns to the plant after delivering a load, there is almost always unused concrete and/or residue accumulated on the inside of the truck drum and chutes. The system delivers chemically treated system-water to the truck drum to suspend hydration and dilute and rinse the drum contents into the close-circuit reclamation and recycling system.
The process of hydration stabilization can be found in a technical document named xe2x80x9cA Novel Method Of Recycling Concrete Using Extended Life Admixtures.xe2x80x9d Co-authored by Lawrence R. Roberts of W. R. Grace (Conn.) and Seiji Nakamura of K.K. Denka Japan, which was released at the European Ready-Mix Association congress in 1998, the disclosure of which is incorporated herein by reference.
The term system-water may, throughout this document, also be referred to as wash-water, washout fluid, slurry and batch slurry. It should also be noted, that throughout the course of this process description, when the system transfers slurry from the recovery tank to the secondary tank, the nomenclature used to describe the slurry will change from xe2x80x9csystem-waterxe2x80x9d to xe2x80x9cbatch slurryxe2x80x9d. This is intended to clearly define the difference in the intended purpose of the slurry in each part of the process. The sand and gravel are classified out of the drum contents using, for example, a spiral-classifier re-claimer, while the cement and very fine sand report to the primary tank with the circulating system-water.
It is one object of the present invention to provide an improved method for re-claiming and recycling cement into concrete production.
According to the invention, there is provided a method of recycling waste unset concrete materials containing water, aggregates and partially hydrated cement, the method comprising:
providing a recovery tank;
introducing into the recovery tank water and hydration stabilization chemicals to provide a washout fluid including same;
providing a plurality of transit mixer drums, each containing a quantity of waste concrete;
for each transit mixer drum;
transferring from the recovery tank a quantity of the washout fluid from the recovery tank into each transit mixer drum;
mixing the waste concrete in the transit mixer drum and the washout fluid, thereby forming an aggregate slurry;
transferring the aggregate slurry into an aggregate re-claimer so as to separate the aggregate slurry into aggregates and slurry;
and transferring the slurry to the recovery tank;
providing a slurry supply system for supplying the batch slurry to a concrete batching plant for use of the batch slurry in mixing with aggregates and cement to form fresh concrete in the batching plant;
and transferring the batch slurry from the recovery tank to the slurry supply system for use of the batch slurry.
According to one important feature of the invention, the slurry is transferred from the recovery tank to the slurry supply system for use of the slurry as the batch slurry at a predetermined constant density. This allows the batcher to receive batch slurry at a constant condition ensuring that it can be utilized in predetermined batch mixes utilizing the known and constant parameters of the slurry
Preferably the system-water from the recovery tank is mixed with dilution water to reduce a density of the system-water from the recovery tank to the predetermined constant density.
In one arrangement, after dilution the diluted system-water is stored in a secondary tank from which smaller individual batches are drawn as batch slurry for the batch slurry supply system.
Preferably the system-water from the recovery tank is mixed with dilution water in a transfer duct as it is being transferred between the recovery tank and the secondary tank.
In another arrangement, the system-water from the recovery tank is mixed with dilution water in a transfer duct as it is being transferred to the slurry supply system so that it is stored at an elevated density and diluted only when required at the batching plant.
Preferably the density of the slurry is measured while it is in the duct and a rate of supply of the dilution water is increased until the required density is reached whereupon the rate of supply of dilution water is maintained constant. In this arrangement, information can be stored defining the rate of supply for subsequent transfer of system-water so that the required adjustment can be achieved more quickly.
In order to transfer only batch slurry at the required density, the batch slurry is returned to the recovery tank until the required density is reached.
In accordance with another important feature of the invention, the water and hydration stabilization chemicals are introduced simultaneously into the recovery tank at a predetermined calculated ratio.
Preferably the predetermined ratio is determined based upon a target density for the system-water in the recovery tank and preferably all the water and hydration stabilization chemicals are introduced at that set ratio while the density is at or below the target density. This allows a simple calculation and adjustment and control of the supply in that all materials are supplied at that same ratio which is determined by the target or intended density value even though the density may to reach that target until a number of recoveries have been made, following which the density is controlled by addition of further water and chemicals at the same ratio. For example, in order to keep cement hydration suspended for a period of 48 to 72 hours at a target density of 1.15 g/cm3 (20% solids by mass), HSA will need to be added to the water at a ratio of 0.002:1 or 2.00 liters of HSA for each 1000 liters of water. If, alternatively, the density was 1.07 g/cm3 (10% solids by mass), the amount of HSA would change to 0.0015:1 or 1.5 liters of HSA for each 1000 liters of water. These ratios will be scaled in accordance with temperature variations in the system-water.
In order to maintain that target density, the density of the system-water is repeatedly measured and additional water and chemicals at the same set ratio are added when the density exceeds the target density to dilute the system-water to said target density.
The ratio may be calculated including as a calculation factor the temperature of the system-water in the recovery tank and heating and/or cooling may be applied to the system-water to maintain the system-water at said temperature.
In the event that the recovery tank is filled to capacity and the target density is exceeded to an over density, additional chemicals are added without additional water to provide a quantity of chemicals sufficient for said over density.
In accordance with another important feature of the invention, there is provided a sleep mode in which the slurry is to be left in storage for a period of time greater than a working period, in which mode additional chemicals are added without additional water at an amount dependent upon the time period beyond the working period. For example, if the density rises to 1.20 g/cm3 (14% solids by mass), chemical will be added according to the density based on the assumption that the tank is full and the chemical must be added in ratio to that full volume. Up to and including a density of 1.30 g/cm3 (35% solids by mass) the system will add chemical at incremental intervals of one unit of specific gravity across the entire volume of the recovery tank.
The slurry supply system may include a batch tank dimensioned to receive and store a batch of the batch slurry substantially equal to or greater than a required batch for the batch plant.
It is advantageous if the batch tank has a discharge for supply to the batch plant which discharges the slurry at a rate greater than a rate of supply thereto so that the batch can be discharged rapidly into the batch plant for use while the batch tank can be re-filled more slowly using the transfer pump from the secondary tank to the batch tank.
Preferably the secondary tank is dimensioned to hold a quantity of the batch slurry equal to or greater than a series of batches of the batch tank for use of the batch tank repeatedly during a work period, for example one shift or one day, and wherein the secondary tank is filled with the required amount of diluted slurry from the recovery tank for that period. For example, if the batcher requires 125 liters per cubic meter of concrete, and he must batch 300 cubic meters over the course of a work period, then he will need to transfer 37,500 liters of batch slurry to the secondary tank during the work period to fulfill that requirement.
Preferably the batch slurry is stored at a temperature lower than a required temperature for the concrete batching plant and is mixed with hot water to raise the temperature to the required temperature at or prior to the batching plant. In this arrangement the batch slurry can be diluted with hot water to effect heating to the required temperature and to effect reduction in density to the required density.